This disclosure is directed to a borehole television system for use in well boreholes. The BHTV provides an output signal which is indicative of the nature of the borehole. Useful information can be obtained from this. While the term BHTV is used, it does not ordinarily refer to a system which operates where the surface is illuminated with light and surface reflections are then observed. Rather, the surface is illuminated with acoustic pulses and the acoustic pulse return signal is used in some fashion to obtain indication of the surface of the surrounding borehole. This procedure is normally carried out in an open hole condition where the well is filled with drilling fluid (mud is the usual term). The wall is intended to be at a controlled and specific distance from the antenna which transmits and then receives the acoustic pulse. For optimum resolution, the acoustic energy is focused at some specific distance from the logging tool. Sensitivity for the instrument is optimum if the borehole wall is at the focused range from the instrument. This assumes, of course, that the sonde which supports the instrument is centralized in the hole so that the rotated antenna array is rotating along the centerline axis of the borehole. This also assumes that the hole is circular and that surface irregularities are minimal. These are very nice assumptions which do not always hold true. Rather, there are many instances where the focal distance of the transducer does not match the distance of the sidewall. This event can arise for a multitude of reasons, and it is sufficient to note that perfect focusing simply is not possible with a fixed focus transducer. The wall may be irregular, the hole may be non-circular, or the surface irregularities may be quite large. In addition, the sonde may not be centered in the borehole. For these and other reasons, it is difficult if not impossible to make a BHTV image which conveys all the data and information which is desired when the range to the borehole wall varies considerably. The output which results from an out of focus system cannot be improved if the transmitted pulse was out of focus, so to speak, resulting in a loss of resolution. Accordingly, the BHTV image is not sharply resolved.
The present disclosure sets forth a focused planar transducer providing improved depth of field. This depth of field is accompanied by an increased return signal amplitude. These two improvements enable a sharper image to be obtained from a BHTV system so that surface roughness and other important factors regarding the formations penetrated by the borehole can be understood. With higher resolution, intersected boundaries along the borehole can be observed more readily. The present apparatus thus serves as an improved acoustic BHTV system, markedly improved over that which is set forth in representative U.S. Pat. No. 4,780,857 or that suggested by UK Patent 2,168,569A which first suggests focusing the transducer element to increase resolution and increase the available signal amplitude. The increase in aperture of the transducer also reduces the effect of elliptical holes and decentralization of the tool in the borehole. The new transducer design also allows for more advance signal analysis techniques to be applied. Since the focusing is done electronically, multiple simultaneous focused beams may be formed. By analyzing more than one resolution at a time, a technique similar to differential phase contrast microscopy may be applied to substantially increase the resolution of the travel time measurement. References to this topic may be found in IEEE Transactions on Sonics and Ultrasonics, Vol. SU-29, No. 6, November 1982, p. 321 and entitled Dichromatic Differential Phase Contrast Microscopy. The present system is thus operative at typical acoustic pulse frequencies in the range of about 100 to about 500 kilohertz wherein an acoustic pulse is transmitted normal to the borehole wall. The present apparatus provides more accurate representation of the surface features. Accordingly, the nature or character of the surface can be known more readily.
The present apparatus is briefly summarized as incorporating a circular planar member having multiple grooves cut therein where the grooves are circular and spaced about a common axis. Each ring is connected to its own transmitter and receiver system. Each ring defined by the grooves serves as a transmitter and receiver in its own right. The transmitters are controlled with digital logic such that each one produces a pulse at the appropriate time to cause the acoustic energy to be focused at the desired distance from the transducer. In the receive mode, the signal from each element is supplied to a preamplifier and then to a multi-tap delay line. Since there are N rings (where N is a whole number integer), there are preferably N-1 delay lines and they are selectively tapped so that the outputs of the several delay lines are summed at a summing amplifier to provide an output. The several summed signals provide the received signal focused at the same range as the transmitted signal. Alternately, the transducer may also be dynamically focused by selecting appropriate taps of the delay lines as a function of time. Doing this will cause the focus for the receive signal to be correct for any received signal. By choosing fewer than all of the elements as the receiver, the apparent aperture and therefore the degree of focusing may be changed. Indeed many degrees of focusing are available simultaneously during a measurement cycle. To implement the phase contrast imaging process, the phase difference between a highly focused signal and a less focused signal are compared. The phase difference is an indication of difference in distance between the highly focused signal image and the less focused image. Hence the term phase contrast imaging.